1. Field of the Invention
The present invention relates to stabilised compositions based on alpha-olefin polymers. It relates more particularly to the compositions based on propylene polymers stabilised against the degradation produced in contact with oxidising liquids and particularly in contact with those containing washing products.
2. Background of the Art
It is known that all polyolefins have the disadvantage of being degraded as a result of oxidation phenomena which are accelerated by various factors. This degradation results in an increase in colour and a decrease in the mechanical properties of the articles manufactured from these polyolefins.
The addition of stabilising systems containing phenolic antioxidants to the polyolefins is a known means of effectively counteracting the appearance of these phenomena.
Usually, these stabilising systems additionally contain other compounds which frequently increase synergistically the efficiency of phenolic antioxidants. These may be, in particular, dialkyl esters of a thiodialkanoic acid, such as dilauryl and distearyl thiodipropionate, organic phosphites such as trialkyl phosphites, and cyclic diphosphites derived from pentaerythritol, phosphonites such as tetrakisphenyl diphenylenediphosphonites, as well as certain mixtures of these compounds (see for example SANDOZ Pat. No. BE-A-774,363 and Kunststoffe, vol. 67, 1977, No. 3, pages 151 to 153).
Finally, the stability may be improved further by the addition of metal salts such as calcium stearate.
Some applications of polyolefins, however, involve their being in prolonged contact at a relatively high temperature with particularly agressive media. This is the case, for example, when they are moulded in the form of components intended to equip industrial and domestic appliances, particularly pumps, washing machines and dishwashers, in which they are in the presence of hot oxidising liquids, more particularly those containing washing products and detergents containing extremely oxidising compounds.
Some known stabilising systems counteract with more or less satisfactory efficiency the degradation caused by the contact of polyolefins with some oxidising liquids. Nevertheless, none of these systems have been found capable of efficiently counteracting the degradation phenomena caused by all the types of oxidising liquids, in particular those containing any washing product, be it in solid or liquid form, and whatever its composition and its concentration of oxidising agents.